The high vibratory and tensile stresses experienced by rotating turbo machinery in operation, particularly the blading members of the fan, compressor, and turbine stages in gas turbine engines, make such components susceptible to high cycle fatigue (HCF) and other stress related failure mechanisms such as stress corrosion cracking (SCC). HCF and SCC ultimately limit the service life of these components as prolonged exposure to such extreme operating conditions leads to the development of fatigue cracks in areas of the component subject to high operational stresses. The fatigue life of a component is further limited by the occurrence of foreign object damage (FOD). FOD locations act as stress risers or stress concentrators that hasten the development and propagation of fatigue cracks. FOD, especially along the leading and trailing edges of blading members, significantly reduces the service life of aerospace components.
The potentially catastrophic effects of HCF and FOD require that fatigue-life limited components be periodically inspected for both cracks and FOD. Any damage or cracking found during inspection is assessed and the component is retired from service due to the extent of the damage or else repaired and returned to service. The inspection of parts and the retirement of parts from service adversely impacts both flight readiness and maintenance costs of the aircraft.
Integrally formed components, such as rotors integrally formed with blading members, also known in the industry as blisks (bladed-disks), blings (bladed-rings), and IBRs (Integrally Bladed Rotors), incur significant maintenance and repair costs due to stress related failure mechanisms and FOD. This is a direct result of their integral or unitary design as opposed to more traditional rotating turbo machinery, such as bladed rotors, where individual components of the construct, such as individual blading members, can be separately removed and repaired or replaced when damage is discovered or the component has reached its pre-determined service life.
FOD and stress related cracking in a single blading member of an integrally formed component may directly impact the integrity of the entire component. Because the integrally formed blading members are not readily removable or replaceable in the event of such damage, an entire integrally bladed rotor may be withdrawn from service due to damage confined to a single blading member. The repair and/or replacement of such a complex component is expensive, both monetarily and from a flight readiness perspective.
The need to replace or repair integrally bladed rotating turbo machinery may be significantly reduced if the fatigue strength, FOD tolerance, and resistance to stress related failure mechanisms of new, serviced, and repaired components can be improved or restored to the as-manufactured condition. Common methods of improving the fatigue strength and foreign object damage tolerance of aerospace components include the introduction of residual compressive stresses in critical areas susceptible to damage and fatigue failure such as the edges and tips of blading members. Introducing compressive residual stresses improves the fatigue properties and foreign object damage tolerance of both new and repaired blading members. This decreases operation and maintenance costs and increases the flight readiness of the aircraft in which the component is employed.
One method currently used to introduce compressive residual stresses in the blading members of integrally bladed rotating turbo machinery is laser shock peening (LSP) as disclosed in U.S. Pat. No. 6,541,733. LSP uses a high power laser system to impart compressive residual stresses at discrete locations on both sides of the integrally formed airfoil or blading member. However, LSP processing each blade of an integrally bladed rotor is labor intensive, time consuming, and expensive.
Burnishing, also referred to as deep rolling, is an equally effective, less expensive, and more time efficient alternative to LSP for inducing compressive residual stresses in the surface of a part. Burnishing, particularly ball burnishing as disclosed in U.S. Pat. Nos. 5,826,453, 6,415,486, and 6,622,570, has been shown to effectively increase the fatigue strength and FOD tolerance of aerospace components, such as airfoils and turbine disks, and to substantially mitigate or eliminate stress induced failure mechanisms.
While burnishing is generally well suited for aerospace applications, the geometrical complexity and unitary design of some aerospace components, such as integrally bladed rotors, does not readily permit the use of current, commercially available burnishing tools to introduce compressive residual stresses in the individual, integrally formed blading members. As a practical matter, the complex shape of the blading members and the narrow spacing between individual blading members of the integrally bladed rotor does not provide adequate clearance to permit the use of current tool designs to accomplish the introduction of compressive residual stress.
Accordingly, a need exists for an efficient and cost effective method of imparting residual compressive stresses in the individual blading members of integrally bladed rotating turbo machinery to either improve or restore the fatigue performance and/or resistance to stress related failure mechanisms of the blading members thereof.